The respiratory system takes in the oxygen that is required for cellular respiration within eukaryotic cell's mitochondria and pushes out the carbon dioxide produced.
The oxygen that enters and the carbon dioxide that leaves the organism via the repiratory system is transported within the body via the circulatory system. The circulatory system delivers the oxygen to the cells that are in need of it and picks off the CO2 to carry away from the cells. Hemoglobin is a structure that resides in the center of a red blood cell that is made of four protein strands. It is the location where oxygen attaches to the red blood cell in order to be delivered to the tissues in need. It is also where the carbon dioxide attaches to leave the body. The circulatory system uses arteries to deliver oxygenated blood and veins to transport oxygen-depleted blood.
Meanwhile, the digestive system takes in the food (glucose) and uses the oxygen during cellular respiration to produce energy (in the form of adenine triphosphate, or ATP). It is during cellular respiration that carbon dioxide is produced.